Referring to FIG. 1, a conventional vertical LED includes a sandwich structure formed by an N-type semiconductor layer 1, an active layer 2 and a P-type semiconductor layer 3. Below the P-type semiconductor layer 3, a metal mirror layer 4, a metal buffer layer 5, a binding layer 6, a silicon substrate 7 and a P-type electrode 8 are disposed in sequence. A surface of the N-type semiconductor layer 1 is processed by a roughening treatment for increasing light extraction. An N-type electrode 9 is further provided. By applying a voltage to the N-type electrode 9 and the P-type electrode 8, the N-type semiconductor layer 1 is enabled to provide electrons and the P-type semiconductor layer 3 is enabled to provide holes. Light is produced by the electrons and holes that combine at the active layer 2.
Conventionally, to increase the light extraction efficiency of an LED, the light emitted from the active layer 2 is generally reflected by the metal mirror layer 4. Thus, the metal mirror layer 4 is selected from a silver/titanium tungsten/platinum alloy coating, a silver/titanium/platinum alloy coating, a silver/titanium/tungsten/nickel alloy coating or a silver/nickel alloy coating having high reflection efficiency. Through properties of high reflection efficiency and high thermal stability of the metal mirror layer 4 selected from the above materials, the amount of reflected light is maximized to increase the light extraction efficiency while also maintaining stable electrical characteristics.
However, in the LED, after forming the metal mirror layer 4 below the P-type semiconductor layer 3, the buffer layer 5 and the binding layer 6 need to be formed by further involving numerous semiconductor processes. As a result, the silver in the mirror layer 4 is liable to oxidation in the subsequent processes, such that the reflection efficiency of the mirror layer 4 is degraded to thus lower the light extraction efficiency of the LED.
To solve the above issue, referring to FIG. 2 and FIG. 3, the U.S. Pat. No. 8,766,303, “Light-emitting diode with a mirror protection layer”, discloses a structure including an N-type electrode 10, an N-type semiconductor layer 11, a light emitting layer 12, a P-type semiconductor layer 13, a metal mirror layer 14, a protection layer 15B, a protection adhesive layer 15A, a metal buffer layer 16, a binding layer 17, a permanent substrate 18, and a P-type electrode 19. The protection adhesive layer 15A and the protection layer 15B are formed at one side of the metal mirror layer 14 away from the P-type semiconductor layer 13 (as shown in FIG. 2), and shield a side edge of the metal mirror layer 14.
Alternatively, the protection adhesive layer 15A and the protection layer 15B are formed between the P-type semiconductor layer 13 and the metal buffer layer 16 (as shown in FIG. 3), and shield a side edge of the metal mirror layer 14. Thus, the P-type semiconductor layer 13, the protection adhesive layer 15A, the protection layer 15B and the buffer layer 16 completely shield a side edge of the metal mirror layer 14, so as to prevent the metal mirror layer 14 from oxidation in the subsequent processes.
In the above known technology, the protection layer 15B is selected from a group consisting of titanium dioxide, silicon dioxide, aluminum oxide and tin indium oxide, and features a high stability and sustainable physical properties. It is to be noted that, the protection adhesive layer 15A is formed by titanium, tungsten and chromium, and a metal alloy incorporating these elements. In general, the application of such metal adhesive layer in a normal environment (with temperature between 20° C. and 27° C. and humidity between 50% and 60%), the issue of unsatisfactory adhesion of an oxide can be overcome. However, when exposed to high current density operations in an extreme environment, the protection adhesive layer 15A may not steadily clad the protection layer 15B at the edge of the metal mirror layer 14 due to thermal expansion and water oxidation. As a result, the metal mirror layer 14 inevitably becomes oxidized in the subsequent processes to fail the expected application requirements.